DESCRIPTION: (Adapted from the application). The central hypothesis that the applicant claims about his research is that AD is a syndrome in which mutations or polymorphisms in numerous distinct genes lead to altered APP processing or decreased AB clearance, resulting in an imbalance between AB production and removal. He contends that this imbalance causes the gradual accumulation of non-filamentous and then filamentous polymers of AB as a necessary step which precedes the glial, neuronal and synaptic pathology that produce the dementia. He states that studies from many labs suggest that the aggregation (assembly) state of AB is a critical determinant of its cytotoxicity. And also, that all previous studies of aggregation have employed high (uM-mM) concentrations of synthetic peptides of a specified length in nonphysiological conditions, leading both to substantial inconsistencies among studies and concerns about their relevance to AB aggregation in the human brain. The applicant's laboratory has previously studied oligomers of naturally secreted AB that form under biologically relevant conditions and concentrations (pM-nM) in living cultures of neural and non-neural cells. He states that this endogenous AB aggregation system can be used to advantage, to examine very early steps, in AB polymerization while avoiding many of the limitations of synthetic peptide studies in the test tube. Based on preliminary data, the proposal is to analyze systematically endogenous AB oligomers as to their biochemical nature, stability and fate, and their regulation by molecules (proteins, drugs) that enhance or retard their formation. the aims in this project are as follows: 1) Characterize extracellular oligomer formation and fate under normal and FAD conditions. 2) Determine whether AB oligomerization actually begins intracellularly; and if so, in which subcellular compartment, and whether oligomer formation varies by cell type (eg, neuronal vs. endothelial). 3) Screen for and identify naturally occurring pro-aggregating proteins released by cells. 4) Use 125I- and 35S-AB assays to search for and characterize compounds that retard natural oligomer formation. These experiments are intended to address the mechanism and inhibition of native AB aggregation in living cells, which is seen as a major potential therapeutic target in AD.